Vehicular suspensions are well known for providing a smoother and more comfortable ride for driver and passengers. A typical embodiment utilizes springs, providing support for the suspension; and shock absorbers, providing damping, both of which are operatively attached between the axle and vehicular chassis frame. When traveling upon a rough road or off-road terrain, the vehicle wheels and axles begin to oscillate up and down; the frequency increasing with increased velocity of the vehicle. This oscillation is facilitated by the vehicle's suspension system which determines the extent of vertical travel distance and degree of driver comfort.
Additionally, when a vehicle is heavily loaded, the supporting springs compress downward upon the rear end causing the front end of the vehicle to rise. This downward movement upon the rear axle reduces the vertical travel range of the rear suspension substantially causing any installed rear bump-stops or limiters to become activated which could detrimentally affect occupant ride quality. The rear end sag also negatively impacts steering control and handling as the vehicular load is not properly distributed between front and rear suspensions.
Furthermore, pick-up truck manufacturers presently are designing trucks with the ride quality of a sedan in an effort to attract a broader customer base. To achieve this additional level of ride comfort, leaf spring-packs have been substantially lengthened. The drawback is that longer spring-packs do not support as heavy a load as do shorter spring-packs. Also, longer spring-packs are susceptible to increased lateral twisting which translates into an increased likelihood of body roll. Many drivers do not carry substantial loads and these concerns do not become an issue. However, for vehicles which do carry heavy loads, the longer spring-packs become a detrimental feature and a suspension enhancement becomes a necessity.
An example of a prior art suspension enhancement is a product marketed by Timbren Industries Inc., Ajax, Ontario, Canada, which is illustrated in FIG. 1. This suspension enhancement comprises an extended hollow cylindrical-shaped piece of resilient rubber 10 which replaces the factory bump-stop and is attached on the top end to the vehicular chassis frame (not shown) and is not in contact with axle A when an upward force is not applied to the axle. However, when a sufficient upward force F is applied to axle A, it will displace in the upward vertical direction and compress rubber 10 as shown by the dashed lines of FIG. 1. This compression of hollow rubber body 10 provides additional suspension support. The rubber is not attached to the axle because oscillation in the downward direction would undesirably stretch the rubber piece causing it to fatigue and prematurely fail. As a result, this suspension enhancement addresses only the upward stroke of the axle. However, undesired increased downward rebound occurs which requires increased damping from the shocks. Also, due to its construction from high durometer rubber, the engagement is typically harsh.
A second example of a prior art suspension enhancement is a product marketed by Air Lift Company, Lansing, Mich. This suspension enhancement comprises an inflatable bellows operatively positioned and attached on the top end to the vehicle chassis frame and on the bottom directly to the axle or via the suspension spring. The pressure within the bellows is adjustable.
Although ride comfort is enhanced by the air bag being in continuous operative contact with the axle and frame, this embodiment suffers from a number of problems. One is that the internal bellows pressure will reduce over time as a result of leakage. Another problem is that the bellows suspension enhancement systems are susceptible to rupture failure when not properly maintained. Under-inflation can lead to excessive wear internally and eventual failure by rupture of the air bladder. Over-inflation can cause high point loading on the frame and lead to possible frame damage. Over inflation also detracts from the factory-installed suspension and often results in a hard, uncomfortable ride. Finally, although the air suspension enhancement is operatively attached on the top to the frame and on the bottom to the axle and can handle compression loads for a limited period of time without maintenance, downward extension of the axle places a stress upon the air bag system for which it was not designed. Typically, air bag systems are not recommended for stretching in excess of 15% of its length.
Polyurethane elastomers have been used for many years in applications requiring increased deformability and reduced compression hardness. A process for production of cellular polyurethane is described in U.S. Pat. No. 4,735,970. Cellular polyurethane elastomers, such as cellular VUKOLLAN®, manufactured under license from Bayer Aktiengesellschaft, has also been used in the automotive sector; particularly for use in the manufacture of struts which provide desirable progressive spring characteristics.